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A room temperature sub-ppm H 2 S sensor based on RF sputtered SnO 2 : NiO thin film has been demonstrated. For this, SnO 2 thin films have been modified with ultrathin layers (30 nm) of NiO. These films were characterized using Scanning electron Microsopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Gas sensing properties of these films were systematically investigated and compared with those of pure SnO 2 and NiO films. Sensing studies revealed that these films are highly sensitive to H 2 S at room temperature. The sensor response exhibited by NiO modified SnO 2 film, is 9 and 415 times greater than that exhibited by pure SnO 2 and NiO films, respectively. The sensor response in case of SnO 2 :NiO is due to contribution from two different mechanisms- (i) oxidation reaction between adsorbed oxygen and H 2 S and (ii) destruction of p-n junctions due to conversion of Nickel Oxide to metallic Nickel Sulphide (product of chemical reaction between NiO and H 2 S). Presence of XPS sulphide peaks at binding energies of ∼163 eV corresponding to S-2p 3/2 and S-2p 1/2 , on H 2 S exposure confirms the formation of Nickel Sulphide. Present experiments proved the ability of these films to detect low concentrations of H 2 S i.e. parts per billion reproducibly. These films were also found to be stable for extended period of time, hinting at commercial viability of the sensor. [ABSTRACT FROM AUTHOR]

Gas sensing properties of ZnO nanobelts synthesized using carbothermal reduction method has been investigated. At room temperature (28 °C), the sensor films exhibit an appreciable response towards H 2 S and NO and response of these two gases were studied as a function of concentration. For NO the sensor films exhibit a complete reversible curve for the concentration range between 1 and 60 ppm. However, for H 2 S a complete recovery was obtained for concentration <5 ppm and for higher concentration a partial recovery of the baseline resistance was observed. The reason for the incomplete recovery was investigated using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) studies of the sample before and after the H 2 S exposure. After exposure, appearance of an additional peak at 26.6° corresponding to the formation of ZnS was observed in XRD. Formation of additional phase was further corroborated using the results of XPS. H 2 S exposure causes decrease in the intensity of O 1s peak and appearance of sulphide peaks at binding energies of 162.8 and 161.8 eV corresponding to S-2p peaks – 2p 3/2 and 2p 1/2 , confirms the formation of ZnS upon exposure. [ABSTRACT FROM AUTHOR]

Room temperature gas sensing properties of CuO nanowires synthesized by thermal oxidation of copper foils was studied in different configurations: (i) isolated nanowires aligned between two electrodes, (ii) as grown CuO foil consisting of nanowires and (iii) CuO nanowire films. Sensors were studied for response to different gases. Different sensors showed qualitatively different response on exposure to H2S. Isolated nanowires showed high sensitivity, (~200% for 10 ppm of gas) and fast response (30 s) and recovery times (60 s). In these samples, the resistance mainly decreased on exposure to H2S (though a small initial increase was observed). In CuO foils, resistance increased for low concentrations (5-10 ppm) but decreased at high concentrations. In the case of CuO nanowire films, resistance only increased on exposure of H2S (upto 400 ppm). Since CuO is a p-type semiconductor, on exposure to H2S an increase in resistance is expected due to oxygen adsorption related process. Decrease in resistance in some of the sensors was understood in terms of reaction of CuO with H2S resulting in the formation of CuS. [ABSTRACT FROM AUTHOR]

Heterojunction interfaces play a very important role in improving the gas sensing properties. In our previous study through direct current (dc) measurements we showed that a synergetic balance of depletion width at ZnO-rGO interface with the Debye length leads to the improved selective NO 2 response. In this paper, impedance spectroscopy is used to probe the NO 2 sensing behaviour of ZnO-rGO heterostructure films as the processes of different time constants in gas/ solid interactions can be distinguished by varying the AC frequency. The Nyquist plots for ZnO-rGO films measured under ambient as well as after NO 2 exposure shows that sensor response is governed by the variation in potential barrier of depletion region formed at the p-n junction between ZnO and rGO. To understand the adsorption mechanism, experimental response data measured at different NO 2 concentrations were fitted using suitable adsorption kinetic models. Elovich model is found to be apt for lower concentrations where energetically different sites are involved in the adsorption process while higher concentrations are better modelled with pseudo first order kinetics that is independent of the surface coverage. [Display omitted] • Demonstrated NO 2 sensing of ZnO-rGO nano-composite by impedance spectroscopy as an alternate approach. • The role of interface has been investigated by impedance spectroscopy and work function study. • Investigated adsorption kinetics, which permits an alternate way to determine NO 2 concentration from sensor response rate. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, significant interest has emerged in the synthesis of nanoscale materials owing to their superior and enhanced functional properties. And metal oxides based nanostructures are considered to be the most attractive class of materials for realizing functional nanodevices. Of the various nanostructures, nanowires (NWs) are particularly useful for gas sensing application as they offer various advantages. These includes high surface area-to-volume ratio, pathway for electron transfer (length of NWs), enhanced and tunable surface reactivity implying possible room temperature operation, faster response and recovery time, dimensions comparable to the extension of the surface charge region, relatively simple preparation methods allowing large-scale production, convenient to use, ease of fabrication and manipulation, high integration density, smaller size and low power consumption. All the above mentioned features definitely make NWs a promising candidate for nanosensors. The present review deals with the progress made toward the effective use of NWs in particular metal oxide based for achieving superior sensing performance. The promising approaches contributing toward the further development of this field have been discussed and elaborated. Various issues related to successful utilization of NWs for commercial sensor application is discussed citing examples from the literature. In particular, different sensor configurations like single-NW based, multiple-NW based, NW films/as-grown NWs films have been critically addressed. The important steps taken toward commercialization of ultimate sensor device and the major obstacles involved are also discussed. [Copyright &y& Elsevier]

Abstract: Random networks of ZnO nanowires (ZnO-NWs) have been fabricated on the Si/SiO2 substrates by hydrothermal process using ZnO-nanoparticles as seed. The ZnO-NWs networks have been modified by depositing ultrathin CuO layers (nominal thickness: 10nm) using thermal evaporation of Cu followed by oxygen annealing at 600°C. The results of photoluminescence measurements on ZnO-NWs and CuO:ZnO-NWs networks suggest formation of a p–n junction between p-CuO and n-type ZnO. The gas sensing characteristics of both ZnO-NWs and CuO:ZnO-NWs networks have been investigated. It has been demonstrated that CuO:ZnO-NWs network has selective response toward H2S with a sensitivity of ∼40 (for 10ppm) at an operating temperature of 200°C. A sensing mechanism based on the destruction of p–n junctions due to the formation of metallic CuS (product of chemical reaction between CuO and H2S) has been proposed. [Copyright &y& Elsevier]

The nano crystalline hexagonal strontium ferrite nanoparticles SrFe 12 O 19 were synthesized successfully by chemical co-precipitation method. Thin films of strontium ferrite were prepared on glass substrate and characterized by various techniques such as XRD, SEM, TEM, EDS, UV-spectroscopy and FTIR. XRD pattern revealed the phase transformation of M-type hexa-ferrite with the minimum crystallite size of 18 nm. Uniform macroporous surface structure of the film was exposed by SEM images. Existence of iron, strontium and oxygen in the material was confirmed by EDX. Optical characterization of the material was done by UV-Spectroscopy and band gap was found as 3.2 eV. The liquefied petroleum gas (LPG) sensing behavior of strontium ferrite film was investigated at room temperature. The variations in electrical resistance of the film were measured with the exposure of LPG with respect to time as a function of concentration (0.5–5 vol.%) of LPG. The maximum value of sensitivity for these films was found 7 and maximum sensor response was 602.23. [ABSTRACT FROM AUTHOR]

Perovskite barium titanate (BaTiO 3 ) nanocomposite thin film has been prepared using spin coating technique. Surface morphological, structural, compositional and optical properties of the film were investigated by various techniques such as SEM, XRD, TEM, EDS, UV-Spectroscopy, Raman spectroscopy and FTIR. SEM images showed the macroporous cubic structure of the film. The minimum crystallite size was evaluated as 11 nm by XRD and confirmed by TEM. Band gap of BaTiO 3 was evaluated as 3.9 eV. Raman Spectroscopy confirmed the formation of BaTiO 3 at 540 cm −1 . LPG sensings were carried out for vol.% 0.5–4 of LPG and sensor was found robust for the detection of LEL for LPG also. [ABSTRACT FROM AUTHOR]

Abstract: Hierarchical heterostructures consisting of W18O49 nanowires grown on SnO2 nanowires have been prepared by sequential thermal evaporation of tin and tungsten under partial oxygen atmosphere. Sensors made using isolated heterostructure nanowires showed superior selectivity for the detection of chlorine in comparison to isolated pure SnO2 nanowires as well as mat type films prepared using heterostructure nanowires. Improved response compared to mat type films arises due to different sensing mechanisms of chlorine arising from its adsorption or replacement of oxygen. On the other hand better selectivity in comparison to isolated SnO2 nanowires has been attributed to transfer of electrons across hetero-junction from W18O49 to SnO2 and lower sensitivity of W18O49 to H2S. The results show potential of tailored hierarchical nanoheterostructures for the fabrication of gas sensors. [Copyright &y& Elsevier]